The Qutb Minar Complex

The Qutb Minar Complex is the site of the first Muslim capital of Delhi, dating from the late 12th century. The invading Muslims overthrew an older Hindu kingdom and reused their stones, as well as stones from Jain structures, to build the first mosque in India. Within the courtyard of the mosque is an iron pillar with a Sanskrit dedication to Chandragupta II, who ruled the Gupta Empire (considered the "Golden Age" of India) from c. 375-413. This iron pillar was probably originally in Bihar and may have been moved by the rulers of the Hindu kingdom that existed here prior to the Muslim conquest. It's rare to see things of this great age in such fantastic condition.
Dominating the site is the Qutb Minar itself, a 200-ft-high tower constructed between the 12th and 14th centuries. It is the tallest stone tower in India, and is notable for its stylistic affinity to Afghan towers due to the origins of its builders.
In addition to the mosque and tower are other structures, including tombs of kingdom officials as well as some of the early sultans themselves. There is also the foundation of the Alai Minar, which was to be twice as high as the Qutb Minar, but was never finished. Now it is just a crumbling (though still imposing) stump.
For a "capital", the complex is surprisingly small and easy to wander. It's a really fascinating place and should be on every tourist's itinerary for Delhi.